1. Field of the Invention
The present invention relates to resonators and more particularly to unstable laser resonators especially adaptable to high power chemical laser applications.
2. Description of the Prior Art
Chemical lasers have been recognized as suitable devices for providing laser radiation at high levels of power. The amount of power producible by such means is directly related to the volume of gain medium which can be controlled. Various previous attempts to increase the amount of power in the output beam from a chemical laser simply involved increasing the volume of the active gain medium. As the demand for power continues to increase, the ability to increase the power level from the conventional laser systems by increasing the dimensions associated with the volume of the active medium reaches a predictable limit due to a natural phenomenon known as superfluorescence. As used herein, superfluorescence means the phenomenon in which oscillations can be initiated from spontaneous emission and sustained without the benefit of positive optical feedback structure such as the reflecting surfaces of mirrors in a resonator. In effect, should the maximum dimension in any direction through a gain medium having a population inversion exceed some critical dimension dictated by superfluorescence, spontaneous emissions in the gain medium cause an uncontrolled depopulating of the medium resulting in random bursts of energy in random directions and reduction in the power level of the output beam. Therefore, some conceptually different apparatus arrangement are required if the usable power extractable from such devices is to be increased.
One modification to conventional resonator design which allows a large volume of gain medium in the shape of a disc in the resonator is taught by Casperson as disclosed in "Properties of Radial Mode CO.sub.2 Laser," IEEE Journal of Quantum Electronics, Vol. QE-9, No. 4, Apr. 1973, pages 484-488. The stable resonator design shown in FIG. 1 of this article can be adapted to various laser systems having high gain to provide an increased volume of gain medium without incurring superradiance, however, the beam of output energy from such a resonator is of poor optical quality for many potential applications because large volume stable resonators tend to oscillate on higher order transverse modes and to exhibit poor transverse mode discrimination.
A practical resonator design which permits a very large volume of gain medium without sacrificing transverse mode discrimination is taught in U.S. Pat. (application Ser. No. 533,376) entitled Unstable Split Mode Laser Resonator, filed on Dec. 16, 1974 and held with the present invention by common assignee. In the split mode resonator, two separate active gain regions are interconnected with a common folding mirror and a common coupling mirror to produce an output beam having a power which is essentially twice the maximum power which was previously available. This split mode concept is a useful teaching, however, it also has shortcomings. For example, some applications have a requirement for a laser beam at a power level even higher than is possible with the split mode teaching. Also, the split mode resonator has a power distribution in the far field which is somewhat of a departure from strict diffraction limited characteristics.